A license can only be attached to one account at a time. So, the seller needs to unattach the license from his profile and you will be able to enter the license key in your profile and the original seller has lost the ability and the official license to use Lumion.

Also, in most countries an email statement email in combination with your bank transfer or other payment is legal documentation of your purchase. At least it's just as valid if not more valid than a receipt.